Document $: in the tutorial

This change re-words some of the prose so that the label syntax is spoken of in _unfamiliar_ terms rather than _alien_ as it is valid syntax and not some svelte-specific feature/functionality.

site/content/tutorial/02-reactivity/02-reactive-declarations/text.md

@@ -11,7 +11,7 @@ let count = 0;
 $: doubled = count * 2;
 ```
 
-> Don't worry if this looks a little alien. It's valid (if unconventional) JavaScript, which Svelte interprets to mean 're-run this code whenever any of the referenced values change'. Once you get used to it, there's no going back.
+> Don't worry if this looks unfamiliar. It's valid JavaScript, which Svelte interprets to mean 're-run this code whenever any of the referenced values change'. Once you get used to it, there's no going back. For those that are curious, `$:` is [label syntax](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/label).
 
 Let's use `doubled` in our markup: